The invention relates generally to gas turbine technology. More particularly, the invention relates to gas turbine shutdown using fuel-to-air ratio and/or rotor speed control.
Current approaches to gas turbine shutdown reduce fuel flow as a function of time. This process may be accomplished by determining a fuel flow at full speed of the turbine, and then reducing the fuel flow (F) over time (dF/dt)). Once the fuel flow and/or rotor speed is sufficiently low (e.g., 20% of full speed) for the particular turbine, the fuel flow is stopped, and the turbine decelerates to a minimum speed, completing the shutdown of the turbine.
One challenge posed by the above-described approach is that it leads to a large variation in shutdown behavior, which cannot be readily controlled. More specifically, shutting down a gas turbine by reducing fuel flow as a function of time does not ensure a direct relationship between fuel flow and the turbine's operating speed. Rather, significant variations in the rotor speed versus time result from differences in various losses, different turbine conditions, etc. The variation in rotor speed produces significant differences in the fuel-to-air ratio because air intake is a function of rotor speed, and fuel flow is not directly related to speed. Consequently, reducing fuel flow does not necessarily allow for control of how the turbine is operating. In particular, the uncontrolled and varying fuel-to-air ratios may result in variations in firing temperatures, exhaust temperatures and emission rates. This approach also renders a number of characteristics unpredictable such as the time it takes to decelerate from full speed to ‘turning gear speed’, i.e., the speed at which the rotor must be continually turned by an outside source so as to prevent bowing of the rotor. It also makes it difficult to accurately predict part life since the time and temperatures that the part is exposed to in actual operation varies. Additionally, the variation in shutdown behavior will have an effect on turbine clearances, and can result in additional conservatism in the required clearances and a resulting loss of gas turbine performance.